Someone will be along soon with better advice but in your position, I'd encourage DH to set up some kind of pension for himself with at least part of that money - there are ways that self-employed people can do that - or at least start an ISA of some kind since the interest on that is tax free. The idea of stocks & shares is scary to me but longer term has been proven to give a better return than a cash ISA.
I wouldn't spend all the inheritance on the mortgage since when you are ready to retire, you can sell the property to pay off anything you might still owe and then, ideally, also have a lump sum for somewhere else. If you've put all the inheritance into the mortgage, all you will have is the property: you won't have any savings/pensions and when you sell the property you may lose money due to capital gains tax.
You say DH's business failed but what is he doing now? If he is working, he should be contributing to a pension so he could put some of the inheritance into that pension since you are currently allowed to contribute up to 30K extra into your pension each tax year.
Did he work for a company before he started the business? If so, he might have saved into their pension scheme so will have a little coming each month from that when he retires. Lots of people have several pensions coming from their different jobs over the years. Make sure you are the named beneficiary on all his pensions since they pass outside of a will. When my DH died, he had named me as the primary beneficiary but had designated a small percentage to each of our DDs. (MN is full of threads where the widow discovers that her DH has named someone else to receive his pension!)
If you are currently working, I'd look at maximising your pension contribution if you can afford a little less take-home pay each month. Does your employer have any helpful schemes like salary sacrifice? Also, if you have any savings, you are currently allowed to contribute up to 30K extra into your pension each tax year.